1. Field of the Invention
The present invention relates to an image sensing apparatus that senses still images and moving images and a control method therefore, and more particularly, to an image sensing apparatus having an electronic zoom function, and a control method therefor.
2. Description of the Related Art
Image sensing apparatuses, notably digital cameras and digital video cameras, that form an optical image on an image sensing element, perform various signal processes on the image data obtained by photoelectric conversion, and output a visual image of the object field on a liquid crystal monitor, and at the same time record on a recording medium, have been proposed. The recorded image data is either a still image, which uses substantially all of the data of pixels located within the effective area of the image sensing element, or a moving image, which uses only data obtained by thinning and/or averaging the data of pixels located within the effective area of the image sensing element such that the centers of gravity of the pixels become uniform.
In addition, among image sensing apparatuses, as means for setting the angle of view of the subject to be photographed, there are those that have an optical zoom achieved by a zoom lens and those that have an electronic zoom achieved by processing the image data obtained from the image sensing element. Electronic zoom methods are of two general types. One is a method involving manipulating the drive method used to scan the image sensing element, as described, for example, in Japanese Patent Laid-Open (JPA) 2002-314868. The other is a method involving enlarging the image obtained from the image sensing element, as described, for example, in Japanese Patent Laid-Open (JPA) H08-009221 and Japanese Patent Laid-Open (JPA) H09-033793. Moreover, as described for example in Japanese Patent Laid-Open (JPA) 2002-314868 and Japanese Patent Laid-Open (JPA) H09-033793, a proposal to achieve a wide range of an angle of view setting by combining optical zoom and electronic zoom has also been made.
Further, Japanese Patent Laid-Open (JPA) 2002-314868 also discloses a higher-resolution electronic zoom achieved by changing the reading region using an image sensing element that employs an X-Y address type scanning method.
Moreover, Japanese Patent Laid-Open (JPA) 2003-125270 proposes carrying out electronic zoom by fixing the reading area of the image sensing element and performing enlarging processing on the image data obtained from the image sensing element until a prescribed zoom ratio is reached, and changing the reading area of the image sensing element depending on the magnification if the prescribed magnification is exceeded so as to reduce deterioration in picture quality.
The number of pixels in the image sensing elements used in these image sensing apparatuses has increased every year, and recently some apparatuses have image sensing elements with more than 10 million pixels. As pixel density has increased the resolutions of still images has improved but the time required for image readout has lengthened.
At the same time, with an electronic viewfinder (EVF), which refreshes the image at predetermined periods of time and outputs the refreshed image to the liquid crystal monitor, and during moving image recording, an image of a number of pixels that is determined by the number of pixels in the liquid crystal monitor or by the specifications of moving image recording is acquired at predetermined periods of time. As the number of pixels in the image sensing element increases, the need arises to thin and/or to average more pixels in order to obtain the image of the number of pixels for the EVF and/or for moving picture recording, lengthening the time needed for readout.
Further, with electronic zoom accomplished by manipulating the image sensing element drive method as in Japanese Patent Laid-Open (JPA) 2002-314868, there is a possibility that the rate at which the image from the image sensing element is refreshed declines in proportion to the electronic zoom magnification ratio. In such a case, in order to maintain the rate at which the image from the image sensing element is refreshed, the drive frequency must be increased in proportion to the zoom ratio of the electronic zoom.
Further, in order to obtain more pixel information from the image sensing element so as to improve the image quality, together with increasing the drive frequency it is also necessary to increase the signal processing frequency so as to process at high speed the image data output from the image sensing element. As a result, overall power consumption also increases.
In addition, an image sensing apparatus using an image sensing element employing the X-Y address type scanning method of Japanese Patent Laid-Open (JPA) 2002-314868 has the following problem: Specifically, combination of using electronic zoom that changes the image sensing element read drive method at each predetermined zoom ratio and electronic zoom that enlarges the image data obtained from the image sensing element in accordance with designated zoom ratio makes it impossible to display an image smoothly when the image sensing element read drive method is changed.